Rapid development of wireless communication technology is followed by depletion of frequency resources. Almost all frequency bands are exclusively allocated to a″ number of respective wireless communication services, with few frequency bands left to be allocated to new wireless communication services to be developed in the future.
However, frequency bands allocated to some wireless communication services are not used efficiently. For example, cellular network bands are overloaded in most parts of the world, but amateur radio or paging frequency bands are not.
One of technologies discussed recently to solve the problem of insufficient radio resources is Cognitive Radio (CR) technology, which searches for radio resources having degraded utilization efficiency and shares frequency radio resources without interference with existing systems, which have been assigned corresponding frequencies and used them, thereby improving radio resource efficiency.
In the CR technology, a user formally licensed to use a specific frequency band is referred to as a primary user, a licensed user, or an incumbent user, and a user not licensed to use a specific frequency band is referred to as a secondary user or an unlicensed user.
A factor requiring the most important consideration in the CR technology is the problem of interference of unlicensed users with licensed users. One of the problems that need to be solved to guarantee high spectrum efficiency without interference with licensed users is transmit power control of CR devices of unlicensed users. That is, every CR device must have a transmit power control function which guarantees reliable communication while avoiding interference with any licensed user.